Direct Sun measurements of NO2 column abundances from Table Mountain, California: Intercomparison of low- and high-resolution spectrometers

The NO2 total column abundance, C-NO2, was measured with a direct Sun viewing technique using three different instruments at NASA Jet Propulsion Laboratory's (JPL) Table Mountain Facility in California during an instrument intercomparison campaign in July 2007. The instruments are a high-resolution (similar to 0.001 nm) Fourier transform ultraviolet spectrometer (FTUVS) from JPL and two moderate-resolution grating spectrometers, multifunction differential optical absorption spectroscopy (MF-DOAS) (similar to 0.8 nm) from Washington State University and Pandora (similar to 0.4 nm) from NASA Goddard Space Flight Center. FTUVS uses high spectral resolution to determine the absolute NO2 column abundance independently from the exoatmospheric solar irradiance using rovibrational NO2 absorption lines. The NO2 total column is retrieved after removing the solar background using Doppler-shifted spectra from the east and west limbs of the Sun. The FTUVS measurements were used to validate the independently calibrated measurements of multifunction differential optical absorption spectroscopy (MF-DOAS) and Pandora. The latter two instruments start with measured high-Sun spectra as solar references to retrieve relative NO2 columns and then apply modified Langley or "bootstrap" methods to determine the amounts of NO2 in the references to obtain the absolute NO2 columns. The calibration offset derived from the FTUVS measurements is consistent with the values derived from Langley and bootstrap calibration plots of the NO2 slant column measured by the grating spectrometers. The calibrated total vertical column abundances of NO2, C-NO2, from all three instruments are compared showing that MF-DOAS and Pandora data agree well with each other, and both data sets agree with FTUVS data to within (1.5 +/- 4.1)% and (6.0 +/- 6.0)%, respectively.